The present invention relates generally to magnetic thin films for recording information, and, more particularly, to the use of a two layer magnetic thin film to increase coercivity and/or decrease noise as compared to either layer by itself.
Magnetic media are widely used in the computer industry. The media can be locally magnetized by a write transducer, or write head, to record and store information. The write transducer creates a highly concentrated magnetic field which alternates direction based on bits of the information being stored. When the local magnetic field produced by the write transducer is greater than the coercivity of the recording medium, then grains of the recording medium at that location are magnetized. The grains retain their magnetization after the magnetic field produced by the write transducer is removed. The direction of the magnetization matches the direction of the applied magnetic field. The magnetization of the recording medium can subsequently produce an electrical response in a read transducer, allowing the stored information to be read.
The computer industry continually seeks to reduce size of computer components and to increase the speed at which computer components operate. To this end, it is desired to reduce the size required to magnetically record bits of information. It is concomitantly important to maintain the integrity of the information as size is decreased, and disc drives which magnetically store information must be virtually 100% error free. The space necessary to record information in magnetic media is dependent upon the size of transitions between oppositely magnetized areas. It is generally desired to produce magnetic media which will support as small of transition size as possible. However, the output from the small transition size must avoid excessive noise to reliably maintain integrity of the stored information.
In a recording medium with a square hysteresis loop, the width of a recorded transition, a, is predicted to be EQU a=(M.sub.r td/.pi.H.sub.c).sup.0.5
wherein
M.sub.r is remanent magnetization; PA1 t is medium thickness; PA1 d is the distance from the write transducer to the medium; and PA1 H.sub.c is medium coercivity.
The transition widens with M.sub.r as a result of the fact that the magnetic field existing on one side of a transition affects magnetization on the other side of the transition. The transition narrows as H.sub.c is increased, because with high coercivity, the medium can resist the transition broadening due to the neighboring fields. The magnetic field produced by the write transducer, or head field gradient, is sharpest near the pole tips of the head. The transition widens with d and t, due to the fact that a poorer head field gradient is obtained within the medium when the particles are a further distance from the head. Smaller head-to-medium spacing and thinner medium both lead to narrower transitions being recorded.
Several material parameters influence the ability of a material to magnetize. Shape anisotropy effects the ease of magnetic recording, as particles are more easily magnetized along the long dimension of the particles. Magnetoelastic anisotropy of a material may effect the ease of magnetic recording. Crystalline anisotropy effects the ease of magnetic recording based on the orientation of crystal structures in the material. In thin films, crystalline anisotropy is the primary means of magnetization. In a disc, grains are more easily magnetized along the plane of the disc because the grains have a preferred crystalline orientation for magnetization lying along the plane. The magnetization results given herein are along the plane of the disc.
Magnetic thin films are a particular type of magnetic media which are commonly used in computer applications. Thin film media typically consist of a layer or film of a magnetic substance deposited over a substrate. The magnetic substance may be a cobalt based alloy, and the substrate may be a nickel-phosphored aluminum or may be silicon or glass based. A relatively nonmagnetic underlayer such as chromium may be used between the magnetic film and the substrate.
Thin-film recording media generally have good magnetic properties for small transitions and high-density recording. Because they are nearly 100% dense (voids at the grain boundaries reduce the density somewhat), they can be made to have a high magnetization. Because of their high magnetization, they can be made quite thin and still provide adequate signal during readback. The small thickness of thin-film media helps to narrow the recorded transition. Thin-film media can also be made quite smooth, helping to reduce head to medium spacing.
It has been theorized that noise in thin-film media increases when polycrystalline films are strongly exchange coupled. Exchange coupling occurs due to the magnetic effect of adjacent grains. Strongly exchange coupled films tend to exhibit irregular zigzag transitions, which produce considerable jitter in the transition position relative to the location where the record current in the head goes through zero. Various suggestions have been made toward reducing jitter noise in thin-film media. For instance, it has been proposed that the introduction of nonmagnetic elements which segregate to the grain boundaries during deposition. together with careful control of the sputtering conditions, may achieve a porous microstructure at the grain boundaries and reduce exchange coupling and transition jitter. A thin-film with low noise and low exchange energy characteristics is a primary requirement for overall achievement of magnetic recording performance.
When applied as a thin film, different types of magnetic alloy produce different coercivities and noise values. The coercivities and media noise can also be affected significantly by optimizing the deposition processes and controlling the microstructure of films. For instance, Co-Cr-Ta and Co-Cr-Pt films can be applied to exhibit coercivity values such as in the range of 1800-2600 Oersteds, while Co-Cr-Pt-Ta films applied by the same methods exhibit higher coercivity values such as in the range of 2000-2900 Oersteds. However. the noise produced by the Co-Cr-Pt-Ta films may be twice that the Co-Cr-Ta films. To decrease transition size, it is desired to produce magnetic media which have higher coercivities, while still providing low noise.